1. Field of the Invention
The invention relates to a process for the oxidation of meta-phenoxytoluene by selenium dioxide in the presence of a dehydrating agent to produce meta-phenoxybenzaldehyde containing less than 250 ppm dissolved selenium. The invention relates to a process whereby the benzaldehyde can be recovered from the reaction mixture as the bisulfite addition product at the rate of 90%/hr. The invention further relates to a stoichiometric selenium dioxide oxidation of meta-phenoxytoluene to yield a selectivity of about 90% or greater based on the toluene.
2. Discussion of the Prior Art
The selective oxidation of an alkyl substituted aromatic compound to a benzaldehyde or phenylalkylketone presents a number of problems. In general, an attempt to oxidize alkyl aromatics selectively will lead to a mixture of alkyl aromatic alcohols, carbonyl compounds (aldehydes and ketones), carboxylic acids and phenols. Attempts to oxidize toluenes selectively to benzaldehydes are especially prone to produce benzoic acids because of the tendency of aldehydes to oxidize to the carboxylic acids. For example, the oxidation potentials to oxidize toluene to benzyl alcohol and benzyl alcohol to benzaldehyde are essentially the same as the oxidation potential to oxidize benzaldehyde to benzoic acid.
Contamination by alkyl oxidation products other than aldehyde is not the only problem. Ring oxidation is another direction an oxidation of toluene can take. While toluene is difficult to oxidize selectively to benzaldehyde, a ring substituted toluene offers additional problems. Those toluenes, having strong activating ortho-/paradirecting groups pose the problem of ring substitution. This is especially true in the case where the toluene is meta-substituted with the strong ortho-/para-directing phenoxy group since not only is the phenoxy group a strong ortho-/para-director but the methyl group is also an ortho-/para-director and the activated positions are common to both, i.e. ##STR1## This makes the benzene ring sites designated p,o and o,o especially prone to the replacement of a hydrogen by a hydroxy group.
Selenium dioxide oxidations of organic compounds are known. Stoichiometric oxidation of alkyl substituted olefins are known to give the unsaturated carbonyl compound while stoichiometric oxidation of certain ketones are known to give the diketone. Early attempts to use a stoichiometric oxidation of toluene to benzaldehyde gave the expected mixture of products, i.e., 35% benzaldehyde and 10% benzoic acid c.f. Sultanov et al, Gen. Chem. (USSR), 16 2072 (1946). The best selectivity (based on toluene) are less than 90% of the benzaldehyde.
Selenium dioxide, oxygen and nitrogen dioxide have been employed to catalytically oxidize dimethylaryl compounds and it was reported that the main products under appropriate conditions were the dialdehydes, c.f. W. D. Vanderwerff et al U.S. Pat. No. 3,637,830. The selenium dioxide oxidation of alkyl aromatics whether stoichiometric or catalytic produce selenium compounds which are difficult to remove from the product and appear to interfere with the recovery of the aldehydes as the bisulfite addition product, e.g. less than 20% of the product will have precipitated as the adduct even after one hour.
The present invention is a process which reduces the amount of dissolved selenium in the final product, allows one to easily and quickly recover the aldehyde and in the case of the stoichiometric oxidation raises the selectivity (based on meta-phenoxytoluene going to m-phenoxybenzaldehyde) to about 90% or greater.